U.S. patent number 6,316,383 [Application Number 09/344,488] was granted by the patent office on 2001-11-13 for moldings based on silica.
This patent grant is currently assigned to Degussa AG. Invention is credited to Helmfried Kraus, Hermanus Lansink Rotgerink, Peter Schinke, Thomas Tacke.
United States Patent |
6,316,383 |
Tacke , et al. |
November 13, 2001 |
Moldings based on silica
Abstract
Moldings based on silica having a hollow cylindrical
configuration with internal reinforcing stays or spokes leading
from an inner wall of a hollow cylinder to the center of the
molding or in the form of miniliths having passageway channels
therethrough, are produced by homogenizing silica with methyl
hydroxyethyl cellulose, wax and/or polyethylene glycol. Water and
optionally aqueous alkaline ammonia solution are added, and the
mixture is subjected to kneading and forming, extruding, optionally
cutting the extrudate to the desired length by means of a cutting
device, drying at a temperature from 20 to 150.degree. C., and
annealing for a period from 30 minutes to 10 hours at a temperature
of 400 to 1200.degree. C. The moldings can be used as catalyst
supports for the production of unsaturated esters from olefins,
organic acids and oxygen in the gas phase and and can be used in
particular for the production of vinyl acetate monomer.
Inventors: |
Tacke; Thomas (Paducah, KY),
Schinke; Peter (Rodenbach, DE), Lansink Rotgerink;
Hermanus (Glattbach, DE), Kraus; Helmfried
(Rodenbach, DE) |
Assignee: |
Degussa AG (Frankfurt am Main,
DE)
|
Family
ID: |
7872087 |
Appl.
No.: |
09/344,488 |
Filed: |
June 25, 1999 |
Foreign Application Priority Data
|
|
|
|
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Jun 26, 1998 [DE] |
|
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198 28 491 |
|
Current U.S.
Class: |
502/232; 106/482;
502/233; 502/245; 502/262; 502/527.16; 502/527.14; 502/439;
502/250; 502/243; 501/133; 428/34.4; 428/34.6 |
Current CPC
Class: |
B01J
21/08 (20130101); C07C 67/055 (20130101); B01J
35/026 (20130101); B01J 19/30 (20130101); C07C
67/055 (20130101); C07C 69/01 (20130101); C07C
67/055 (20130101); C07C 69/15 (20130101); B01J
2219/30416 (20130101); Y10T 428/131 (20150115); B01J
2219/30475 (20130101); B01J 2219/312 (20130101); Y10T
428/1317 (20150115); B01J 2219/30223 (20130101) |
Current International
Class: |
B01J
35/02 (20060101); B01J 35/00 (20060101); B01J
21/00 (20060101); B01J 21/08 (20060101); C07C
67/00 (20060101); C07C 67/055 (20060101); B01J
021/08 (); A47G 019/22 (); B28B 021/00 (); C04B
014/04 (); C04B 035/14 () |
Field of
Search: |
;502/232,233,262,245,243,250,439,527.14,527.16 ;428/34.4,34.6
;106/482 ;501/133 ;264/629,632,634,669,670,638,177.11 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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24 25 058 A1 |
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Dec 1975 |
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DE |
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27 19 543 C2 |
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Dec 1977 |
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DE |
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3803895-C1 |
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Apr 1989 |
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DE |
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3912504-A1 |
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Oct 1990 |
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DE |
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195 38 799 A1 |
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Apr 1997 |
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DE |
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0 004 079 A2 |
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Sep 1979 |
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EP |
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0 464 633 A1 |
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Jan 1992 |
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EP |
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0 634 208 A1 |
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Jan 1995 |
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EP |
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0 807 615 A1 |
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Nov 1997 |
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EP |
|
Primary Examiner: Bell; Mark L.
Assistant Examiner: Hailey; Patricia L.
Attorney, Agent or Firm: Pillsbury Winthrop LLP
Claims
What is claimed is:
1. A molding based on silica, comprising pyrogenic silica in a
hollow cylindrical configuration having a structure selected from
the group consisting of internal reinforcing stays or spokes
leading from an inner wall of a hollow cylinder to a center of the
molding and a hollow cylinder having a multiplicity of channels
forming passageways therethrough,
wherein the molding has a diameter from 1 to 25 mm and a ratio of
height to diameter from 0.2 to 5.
2. A molding according to claim 1, wherein the molding has a wall
thickness within the range from 0.05 to 0.3 times the diameter and
a stay or spoke thickness within the range from 0.05 to 0.3 times
the diameter.
3. A molding according to claim 1, wherein the number of internal
reinforcing stays or spokes or passageway channels is at least
3.
4. A process for the production of moldings based on silica
according to claim 1, comprising:
kneading and shaping the silica;
extruding the kneaded, shaped silica;
optionally cutting the extrudate by means of a cutting device;
drying at a temperature of 20 to 150.degree. C.; and
annealing for a period of from 0.5 to 10 hours at a temperature of
from 400 to 1200.degree. C.
5. A process for the production of moldings based on silica
according to claim 1, comprising:
homogenizing the silica with at least one member selected from the
group consisting of methyl hydroxyethyl cellulose, wax and
polyethylene glycol, with the addition of water and optionally with
the addition of an aqueous alkaline ammonia solution,
kneading and shaping or extruding the moldings,
optionally cutting the moldings by means of a cutting device;
drying the moldings at a temperature from 10 to 150.degree. C.;
and
annealing for a period of from 30 minutes to 10 hours at a
temperature of from 400 to 1200.degree. C.
6. A supported catalyst for the production of vinyl acetate monomer
(VAM), which catalyst comprises, as catalytically active components
on a support, at least one member selected from the group
consisting of palladium and palladium compounds, together with
alkali compounds, and which additionally comprises at least one
member selected from the group consisting of gold, gold compounds,
cadmium, cadmium compounds, barium and barium compounds, wherein
the support is a molding according to claim 1.
7. A process for the production of the supported catalyst according
to claim 6 for the production of vinyl acetate monomer
comprising:
depositing Pd, Au, Cd or Ba metal compounds on a support by
impregnation, spraying, evaporation, immersion or
precipitation,
optionally reducing the metal compounds which are deposited on the
support,
optionally washing in order to remove chloride fractions which may
be present,
impregnating with alkali acetates or with alkali compounds which
under reaction conditions for the production of vinyl acetate
monomer are completely or partially converted into alkali
acetates.
8. A process for the production of the supported catalyst according
to claim 6 for the production of vinyl acetate monomer
comprising:
impregnating the support with a basic solution and with a solution
which contains gold and palladium salts, wherein impregnation is
effected simultaneously or in succession, with or without
intermediate drying,
optionally washing the support in order to remove chloride
fractions which may be present,
reducing insoluble compounds which are precipitated on the support
before or after washing,
drying catalyst precursor which is thus obtained, and
impregnating with alkali acetates or with alkali compounds which,
under reaction conditions for the production of vinyl acetate
monomer, are completely or partially converted into alkali
acetates.
Description
CROSS-REFERENCE TO RELATED APPLICATION
This application is based on German Application DE 198 28 491.8,
filed Jun. 26, 1998, which disclosure is incorporated herein by
reference.
FIELD OF THE INVENTION
This invention relates to moldings based on silica, to a process
for their production, and their use as a catalyst for the
acetoxylation of olefines.
BACKGROUND OF THE INVENTION
Silicas, particularly pyrogenic silicas, are characterized by their
extremely finely divided state and their correspondingly high
specific surface, by their very high purity, by their spherical
particles and by the absence of pores. Due to these properties,
there is an increasing interest in pyrogenic silicas as supports
for catalysts (D. Koth, H. Ferch, Chem. Ing. Techn. 52, 628
(1980)).
It is known from DE-B 21 00 778 that granular materials based on
pyrogenic silicas can be used as catalyst supports for the
production of vinyl acetate monomer.
It is known from DE-A 38 03 900 that cylindrical particles which
have arched end faces and which are based on pyrogenic silicas can
be used as catalyst supports for the production of vinyl acetate
monomer.
A process for the production of pressed parts is known from DE-A 39
12 504 in which aluminum stearate, magnesium stearate and/or
graphite are used as a lubricant, and in which urea as well as
methyl cellulose are used as pore forming agents.
These known pressed parts which are produced with magnesium
stearate are commercially available as Aerosil Tablets No. 350, as
supplied by Degussa. They contain about 0.4% by weight Mg.
Catalyst supports for catalysts for the synthesis of vinyl acetate
monomer are known from EP 0 004 079 which consist of extruded
sections with a star-shaped cross-section or which consist of
ribbed lengths.
Catalysts for the synthesis of vinyl acetate monomer, which
comprise at least one passageway channel with an inside diameter of
at least 1 mm, are known from EP-B 464 633.
DE-A 195 38 799 describes a catalyst support in the shape of a
honeycomb which predominantly consists of SiO.sub.2. According to
Example 1 of said patent, this support has a diameter of 25 mm, a
stay width of 1 mm, a stay spacing of 2 mm, and a length of 150 mm.
After being coated with catalytically active elements, the
resulting catalyst can be used for the production of unsaturated
esters from olefins, acids and oxygen in the gas phase, for the
purification of off-gas contaminated by organic substances, and for
the alkylation of aromatic compounds.
WO 97/36679 also describes a catalyst support in the shape of a
honeycomb, which is coated with SiO.sub.2 and which, after
impregnation with palladium and gold and after activation with
potassium acetate, can be used for the production of unsaturated
esters.
Honeycomb-shaped catalysts are characterized by a very low pressure
drop. However, the use of honeycomb-shaped catalysts in industrial
reactors, particularly in tube bundle reactors, results in problems
which are not inconsiderable, particularly with regard to packing
tube reactors. With tube reactors, it is sometimes necessary to
pack several thousand tubes of an industrial installation with
honeycomb catalysts. In the course of this procedure, it has to be
ensured that the honeycomb bodies do not break down during filling.
Considerable emphasis has to be placed on the avoidance of edge
flow effects, since otherwise the catalysts are not capable of
contributing their full effect. Moreover, honeycomb-shaped catalyst
materials exhibit poor thermal condutivity in a radial direction.
This is particularly disadvantageous in reactions in which there is
considerable evolution of heat, as in oxidation reactions for
example. For the aforementioned reasons, there is currently no
known industrial application in which a tube bundle reactor or
thousands of tubes are operated with honeycomb-shaped catalyst
materials. For this reason, reactors are packed with moldings in
the form of pellets, which likewise exhibit a low pressure
drop.
It is known from EP-B 0 519 435 that SiO.sub.2 can be pressed by
means of binders to produce supports, followed by calcining the
supports obtained and washing the calcined support particles with
acid until cations from the binder are no longer released. In
addition, a supported catalyst, a process for the production
thereof, and the use thereof for the production of vinyl acetate
are also described.
EP-A 0 807 615 describes pressed parts based on pyrogenic silica.
These pressed parts can be used as a catalyst or catalyst support
for the production of vinyl acetate monomer and for the hydration
of ethylene. The pressed parts may be of different shapes, e.g.
cylindrical, spherical or annular, with an outside diameter of 0.8
to 20 mm.
SUMMARY OF THE INVENTION
The present invention relates to moldings based on silica, which
are characterized in that that the supporting geometry consists of
a hollow cylindrical configuration with internal reinforcing stays
or spokes leading from the inner wall of the hollow cylinder to the
center of the molding, or is characterized by a multiplicity of
passageway channels.
The moldings according to the invention can have an outside
diameter from 1 to 25 mm and a ratio of height to diameter of 0.2
to 5. Furthermore, they can have a total pore volume of 0.3 to 1.8
ml/g and a BET specific surface of 5 to 400 m.sup.2 /g.
The SiO.sub.2 content of the moldings according to the invention is
preferably more than 99.0% by weight. The proportion of other
constituents can be less than 0.2% by weight. The moldings
according to the invention can therefore be characterized as being
free from binders. The fines can amount to less than 5% by weight.
The bulk density can range from 100 to 700 g/l.
The present invention further relates to a process for the
production of moldings based on silica, which is characterized in
that silica is subjected to a kneading and working process, and is
then extruded. The extrudate is optionally cut to the desired
length by means of a cutting device, is dried at a temperature of
20 to 150.degree. C., and is annealed for a period from 0.5 to 10
hours at a temperature from 400 to 1200.degree. C.
In one particular embodiment of the invention, the silica can be a
pyrogenic silica. Silica which can be used according to the
invention, which thus includes pyrogenic silica, is described in
Ullmanns Enzyklopadie der technischen Chemie, 4th Edition, Volume
21, pages 451 to 476 (1982).
One preferred embodiment of the invention is a process for the
production of moldings based on silica, which is characterized in
that silica is homogenized with methyl hydroxyethyl cellulose, wax
and/or polyethylene glycol with the addition of water and
optionally with the addition of an aqueous alkaline ammonia
solution, is subjected to a kneading and working process, shaped
and/or extruded. The moldings are optionally cut to the desired
length by means of a cutting device, are dried at a temperature
from 10 to 150.degree. C. and are annealed for a period from 30
minutes to 10 hours at a temperature from 400 to 1200.degree.
C.
Kneaders, mixers or mills which enable good homogenization and
compaction of the mixed material to be effected, such as blade
mixers, fluidized bed mixers, rotary mixers or air jet mixers, for
example, can be used for carrying out the process according to the
invention. In particular, mixers can be used which make it possible
to effect additional compaction of the mixed material, such as
plough mixers, kneaders, pan grinders or ball mills. Mixing and
kneading can also be effected directly in the extruder. The
production of the moldings can be effected in single-screw or
twin-screw extruders, in extrusion presses or in tabletting
machines. The moldings according to the invention are preferably
produced by means of extruders.
In one particular embodiment of the invention, the mixture can have
the following composition before it is shaped:
50-90% by weight silica, preferably 65-85% by weight;
0.1-20% by weight methyl hydroxyethyl cellulose, preferably 5-15%
by weight;
0.1-15% wax, preferably 5-12% by weight;
0.1-15% polyethylene glycol, preferably 5-10% by weight.
The moldings can be annealed at 400-1200.degree. C. for 30 minutes
to 10 hours. The fracture strength, total specific surface and the
pore volume can be varied within certain limits by varying the
amounts of substances used and by varying the pressing
pressure.
The moldings according to the invention can be used either directly
as a catalyst or as a catalyst support.
For use as a catalyst support, the moldings can be brought into
contact with a catalytically active substance after their
production and can be activated, if necessary, by suitable further
treatment.
In particular, moldings made from pyrogenic silica can be used as a
support for the catalyst for the production of vinyl acetate
monomer from ethylene, acetic acid and oxygen.
The moldings according to the invention comprise the following
properties or enable such properties to be obtained:
low pressure drops
low bulk density
relatively large external surface per unit volume of a reaction
vessel
improved mass and heat transfer
comparatively simple packing and emptying of industrial tube bundle
reactors, particularly by comparison with known honeycomb-shaped
catalysts.
The low pressure drop across moldings according to the invention
results, amongst other causes, from their geometric dimensions, due
to which there is an extremely high free surface area over the
cross-section of the moldings and/or a very high voids fraction in
the catalyst packing.
Catalysts can be produced based on moldings according to the
invention which enable higher space-time yields and selectivities
to be achieved.
The invention is explained in greater detail below with reference
to the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-section through a pressed part or supported
catalyst in the shape of a cartwheel;
FIG. 2 is a perspective view of the cartwheel shape shown in FIG.
1;
FIG. 3 shows a pressed part or supported catalyst in the shape of
what is termed a minilith; and
FIG. 4 is a perspective view of the minilith shown in FIG. 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIGS. 1-4 illustrate embodiments of the invention. The maximum
outside diameter d.sub.1 of the cartwheel shaped supports, shown in
FIGS. 1 and 2, and of what are termed miniliths, shown in FIGS. 3
and 4, is preferably 25 mm, wherein the ratio of height h to
outside diameter (h/d.sub.1) can range from 0.2 to 5. The inside
diameter of the moldings is denoted by d.sub.2. The wall thickness
the moldings ((d.sub.1 -d.sub.2).times.0.5) can fall within the
range from 0.05 to 0.3 times the outside diameter. The stay or
spoke thickness of the moldings is denoted by d.sub.3 and can fall
within the range from 0.05 to 0.3 times the outside diameter. The
number of internal reinforcing stays or spokes or passageway
channels can amount to at least 3.
The present invention also relates to a supported catalyst for the
production of vinyl acetate monomer (VAM), which catalyst contains,
as catalytically active components on a support (molding),
palladium and/or compounds thereof and alkali compounds, and which
additionally contains gold and/or compounds thereof (Pd/alkali/Au
system) or cadmium and/or compounds thereof (Pd/alkali/Cd system)
or barium and/or compounds thereof (Pd/alkali/Ba system) or
palladium, alkali compounds and mixtures of gold and/or cadmium
and/or barium, and which is characterized in that the support is a
molding according to the invention.
Potassium compounds, such as potassium acetate for example, are
preferably used as alkali compounds.
The catalytically active components can be present in the following
systems:
Pd/Au/alkali compounds
Pd/Cd/alkali compounds
Pd/Ba/alkali compounds
The supported catalysts according to the invention can be used for
the production of vinyl acetate monomer. For this purpose,
ethylene, acetic acid and molecular oxygen or air are reacted in
the gas phase, optionally with the addition of inert gases, at
temperatures between 100 and 250.degree. C. and at normal or
elevated pressure in the presence of the supported catalysts
according to the invention.
A production process of this type is known from the documents DE 16
68 088, U.S. Pat. No. 4,048,096, EP-A 0 519 435, EP-A 0 634 208,
EP-A 0 723 810. EP-A 0 634 209, EP-A 0 632 214, EP-A 0 654 301 and
EP-A 0 0807 615.
The present invention also relates to a process for the production
of the supported catalyst for the production of vinyl acetate
monomer by depositing Pd, Au, Cd or Ba metal compounds by
impregnation, spraying, evaporation, immersion or precipitation,
optionally reducing the reducible metal compounds which are
deposited on the support, optionally washing in order to remove
chloride fractions which may be present, impregnating with alkali
acetates or with alkali compounds which under the reaction
conditions for the production of vinyl acetate monomer are
completely or partially converted into alkali acetates, in a
suitable sequence, which is characterized in that the support is a
molding according to the present invention.
The present invention further relates to a process for producing a
supported catalyst for the production of vinyl acetate monomer by
impregnating the support with a basic solution and a solution which
contains gold and palladium salts, wherein impregnation is effected
simultaneously or in succession, with or without intermediate
drying, optionally washing the support in order to remove chloride
fractions which may be present and reducing the insoluble compounds
which are precipitated on the support before or after washing,
drying the catalyst precursor which is thus obtained, and
impregnating with alkali acetates or with alkali compounds which
under the reaction conditions for the production of vinyl acetate
monomer are completely or partially converted into alkali acetates,
which is characterized in that the support is a molding based on
silica having a supporting geometry that includes a hollow
cylindrical configuration having internal reinforcing stays or
spokes leading from the inner wall of the hollow cylinder to the
center of the molding or that has a multiplicity of passageway
channels.
The supported catalysts according to the invention can be used for
the production of unsaturated esters from olefins, organic acids
and oxygen in the gas phase.
The catalysts of the Pd/alkali/Au system according to the invention
can be obtained by impregnating the support with a basic solution
and with a solution which contains gold and palladium salts,
wherein the impregnation steps can be carried out simultaneously or
in succession, with or without intermediate drying. The support is
subsequently washed in order to remove chloride fractions which may
be present. The insoluble metal compounds which are precipitated on
the support can be reduced before or after washing. The catalyst
precursor which is thus obtained can be dried in order to activate
the catalyst and can be impregnated with alkali acetates or with
alkali compounds which under the reactions conditions for the
production of vinyl acetate monomer are completely or partially
converted into alkali acetates. In general, the noble metals of
Pd/Au catalysts can be present in the form of a shell on the
support.
For Pd/alkali/Ba catalysts, the metal salts can be deposited by
impregnating, spraying, immersion or precipitation (EP 0 519 436).
The same methods are known for Pd/alkali/Cd catalysts (U.S. Pat.
No. 4,902,823, U.S. Pat. No. 3,393,199, U.S. Pat. No.
4,668,819).
Depending on the catalyst system, the supported catalyst can be
reduced.
Reduction of the catalyst can be effected in an aqueous phase or in
the gas phase. Formaldehyde or hydrazine are suitable for reduction
in an aqueous phase, for example.
Reduction in the gas phase can be effected with hydrogen or forming
gas (95% by volume N.sub.2 +5% by volume H.sub.2), ethylene or
ethylene diluted with nitrogen. Reduction with hydrogen can be
conducted at temperatures between 40 and 260.degree. C., preferably
between 70 and 200.degree. C. Reduction with forming gas (95% by
volume N.sub.2 and 5% by volume H.sub.2) can be conducted at
temperatures between 300 and 550.degree. C., preferably between 350
and 500.degree. C. The catalyst can also be reduced with ethylene
in the production reactor, after activation with alkali
acetate.
The catalyst supports according to the invention advantageously
retain their mechanical strength under the reaction conditions of
the catalytic process, particularly under the effect of acetic
acid.
The production of supported catalysts of the Pd/alkali/Au system on
moldings according to the invention is described in greater detail
below.
The moldings according to the invention are impregnated with a
solution which contains palladium and gold. Simultaneously with the
solution which contains noble metals, or in any desired sequence in
succession, the moldings according to the invention are impregnated
with a basic solution which may contain one or more basic
compounds. The basic compound or compounds serve to convert
palladium and gold into their hydroxides.
The compounds in the basic solution may consist of alkali
hydroxides, alkali bicarbonates, alkali carbonates, alkali
silicates or mixtures of these substances. Potassium hydroxide
and/or sodium hydroxide are preferably used.
Palladium chloride, sodium or potassium palladium chloride or
palladium nitrate, for example, can be used as palladium salts for
the production of the solution which contains noble metals.
Gold(III) chloride and tetrachloroauric(III) acid can be used as
gold salts. Potassium palladium chloride, sodium palladium chloride
and/or tetrachloroauric acid are preferably used.
Impregnation of the moldings according to the invention with the
basic solution has an effect on the deposition of the noble metals
in the support material. The basic solution can be used either
simultaneously with the solution of noble metal or can be used in
any desired sequence with this solution. The moldings according to
the invention are brought into contact with the basic solution and
with the solution of noble metal either simultaneously or in any
desired sequence in succession. When the moldings according to the
invention are impregnated in succession with the two solutions, an
intermediate drying step can be carried out after the first
impregnation step.
The pressed parts according to the invention are preferably first
impregnated with the basic compound. Subsequent impregnation with
the solution which contains palladium and gold results in the
precipitation of palladium and gold in a surface shell on the
molding. The reverse sequence of impregnation generally results in
a more or less homogeneous distribution of the noble metals over
the cross-section of the molding used. When the process is
conducted appropriately, however, catalysts comprising a defined
shell can also be obtained when employing the reverse sequence of
impregnation (see U.S. Pat. No. 4,048,096 for example). Catalysts
which comprise a homogeneous or almost homogeneous distribution of
noble metal generally exhibit reduced activity and selectivity.
Catalysts with shell thicknesses less than 1 mm are particularly
suitable. The shell thickness is influenced by the amount of basic
compound which is deposited on the molding in relation to the
desired amount of noble metal. The higher this ratio is, the lower
is the thickness of the shell which is formed. The quantitative
ratio of basic compound to noble metal compounds which is necessary
for a desired shell thickness depends on the nature of the molding
and on the basic compound and noble metal compounds which are
selected. The requisite quantitative ratio is advisedly determined
by a few preliminary tests. The shell thickness which is present
can easily be determined in such tests by cutting open the catalyst
particles.
The minimum necessary amount of basic compound results from the
stoichiometrically calculated amount of hydroxide ions which are
necessary for the conversion of the palladium and gold in the
hydroxide. As an approximate value, the basic compound should be
used in a 1 to 10-fold stoichiometric excess for a shell thickness
of 0.5 mm.
After the pore volume impregnation process, the moldings according
to the invention can be coated with the basic compounds and with
noble metal salts. If intermediate drying is employed, the volume
of both solutions is selected so that they each correspond to about
90 to 100% of the absorption capacity of the molding used. If
intermediate drying is omitted, the sum of the individual volumes
of the two impregnation solutions has to correspond to the above
condition, wherein the individual volumes can be in a ratio of 1:9
to 9:1 to each other. A volume ratio from 3:7 to 7:3 is preferably
employed, particularly a ratio of 1:1. Water is preferably used as
the solvent in both these cases. Suitable organic or
aqueous-organic solvents can also be used.
The reaction of the noble metal salt solution with the basic
solution to form insoluble noble metal compounds occurs slowly and
is generally complete after 1 to 24 hours, depending on the method
of preparation. Thereafter, the water-insoluble noble metal
compounds are treated with reducing agents. Wet reduction can be
effected, with aqueous hydrazine hydrate for example, or a gas
phase reduction can be effected with hydrogen, ethylene, forming
gas or methanol vapour. Reduction can be effected at normal
temperature or at an elevated temperature, and at normal pressure
or under an elevated pressure, optionally with the addition of
inert oases also.
Before and/or after the reduction of the noble metal compounds, any
chloride which may be present on the molding is removed by
thoroughly washing the molding. After washing, the molding should
contain less than 500 ppm, preferably less than 200 ppm, of
chloride.
The molding which is obtained after reduction as a catalyst
precursor is dried and is subsequently impregnated with alkali
acetates or with alkali compounds which under the reaction
conditions for the production of vinyl acetate monomer are
completely or partially converted into alkali acetates. The molding
is preferably impregnated with potassium acetate. A pore volume
impregnation method is preferably used again here. This means that
the requisite amount of potassium acetate is dissolved in a
solvent, preferably water, the volume of which approximately
corresponds to the absorption capacity of the molding for the
selected solvent. This volume is approximately equal to the total
pore volume of the moldings.
The finished, coated molding is subsequently dried to a residual
moisture content of less than 2%. Drying can be effected in air, or
can also optionally be effected under nitrogen as an inert gas.
The production of supported catalysts of the Pd/alkali/Cd or
Pd/alkali/Ba systems on moldings according to the invention is
effected in a known manner according to the patent specifications
cited above.
For the synthesis of vinyl acetate monomer it is advisable to coat
the moldings with 0.2 to 4 preferably 0.3 to 3%, by weight
palladium, 0.1 to 2, preferably 0.15 to 1.5%, by weight gold and 1
to 10, preferably 1.5 to 9%, by weight potassium acetate, with
respect to the weight of the molding used in each case. These data
are applicable to the Pd/alkali/Au system. In the case of moldings
with a bulk density of 500 g/l, these concentration data correspond
to volume-based concentrations of 1.0 to 20 g/l palladium, 0.5 to
10 g/l gold and 5 to 50 g/l potassium acetate. In order to prepare
the impregnation solution, the corresponding amounts of palladium
and gold compounds are dissolved in a volume of water which
approximately corresponds to 90-100% of the water absorption
capacity of the molding in question. The same procedure is employed
for the preparation of the basic solution.
The cadmium content of the Pd/alkali/Cd catalyst (finished, coated
molding) can generally range from 0.1 to 2.5% by weight, preferably
0.4 to 2.0% by weight.
The barium content of the Pd/alkali/Ba catalyst (finished, coated
molding) can generally range from 0.1 to 2.0% by weight, preferably
0.4 to 1.8% by weight.
The palladium content of the Pd/alkali/Cd or Pd/alkali/Ba catalyst
(finished, coated molding) can generally range from 0.2 to 4% by
weight, preferably 0.3 to 3% by weight palladium.
The potassium acetate content of the Pd/alkali/Cd or Pd/alkali/Ba
catalyst (finished, coated molding) can generally range from 1 to
10% by weight, preferably 1.5 to 9% by weight.
Silicas which have the following physical and chemical properties,
and which are also known by the name AEROSIL.RTM., manufactured by
Degussa-Huls AG (Germany), can be used as pyrogenic silicas:
AEROSIL .RTM. silica OX 50 90 130 150 200 300 380 BET specific
surface m.sup.2 /g 50 .+-. 15 90 .+-. 15 130 .+-. 25 150 .+-. 15
200 .+-. 25 300 .+-. 30 380 .+-. 30 Average size of primary
particles nm 40 20 16 14 12 7 7 Tamped density.sup.1) g/l about 130
about 80 about 50 about 50 about 50 about 50 about 50 Loss on
drying.sup.2) <1.5 <1 <1.5 <0.5 <1.5 <1.5 <1.5
(2 hours at 105.degree. C.) Loss on ignition.sup.2)5) <1 <1
<1 <1 <1 <2 <2.5 (2 hours at 1000.degree. C.)
pH.sup.3) 3.8-4.8 3.6-4.5 3.6-4.3 3.6-4.3 3.6-4.3 3.6-4.3 3.6-4.3
(in a 4% aqueous dispersion) SiO.sub.2.sup.6) % wt. >99.8
>99.8 >99.8 >99.8 >99.8 >99.8 >99.8 Al.sub.2
O.sub.3.sup.6) % wt. <0.08 <0.05 <0.05 <0.05 <0.05
<0.05 <0.05 Fe.sub.2 O.sub.3.sup.6) % wt. <0.01 <0.003
<0.003 <0.003 <0.003 <0.003 <0.003 TiO.sub.2.sup.6)
% wt. <0.03 <0.03 <0.03 <0.03 <0.03 <0.03
<0.03 HCl.sup.6)7) % wt. <0.025 <0.025 <0.025 <0.025
<0.025 <0.025 <0.025 Residue on sieve.sup.4) % wt.
<0.02 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05
(Mocker, 45 .mu.m)
.sup.1) according to DIN 53 194
.sup.2) according to DIN 55 921
.sup.3) according to DIN 53 200
.sup.4) according to DIN 53 580
.sup.5) with respect to the substance dried for 2 hours at
105.degree. C.
.sup.6) with respect to the substance calcined for 2 hours at
1000.degree. C.
.sup.7) The HCl content is a constituent of the loss on
ignition
In order to produce AEROSIL.RTM., a volatile silicon compound is
injected through a nozzle into an oxyhydrogen gas flame formed from
hydrogen and air. Silicon tetrachloride is used in most cases. This
compound hydrolyzes to form silica and hydrochloric acid under the
effect of the water formed during the oxyhydrogen gas reaction.
After leaving the flame, the silica enters what is termed a
coagulation zone, in which the AEROSIL.RTM. primary particles and
primary aggregates agglomerate The product, which at this stage is
present as a kind of aerosol, is separated from the accompanying
gaseous substances in cyclones, and is subsequently post-treated
with moist hot air. The residual content of hydrochloric acid can
be reduced to less than 0.025% by this procedure. Since at the end
of this process the AEROSIL.RTM. only has a bulk density of about
15 g/l, it is subjected to a subsequent vacuum compaction step by
means of which tamped densities of about 50 g/l or more can be
achieved.
The particle size of products obtained in this manner can be varied
by varying the reaction conditions, such as the flame temperature,
the content of hydrogen or oxygen, the amount of silicon
tetrachloride, the residence time in the flame or the length of the
coagulation section, for example.
The BET specific surface is determined using nitrogen according to
DIN 66 131. The pore volume is determined by calculation from the
sum of the micro-, meso- and macropore volumes.
Determination of the micro- and mesopores is effected by plotting
an N.sub.2 isotherm and evaluating the isotherm by the BET, de Boer
and Barret, Joyner or Halenda methods.
The macropores are determined by the Hg penetration method.
The invention is further explained by the Examples given below.
EXAMPLE 1
85% by weight AEROSIL.RTM. 200
5% by weight methyl hydroxyethyl cellulose
5% by weight wax
5% by weight polyethylene glycol
were compacted in a kneader, with the addition of water which had
been made slightly alkaline with an aqueous alkaline ammonia
solution (15 ml of a 32% solution for a 2 kg batch). The kneaded
material was shaped in a single-screw extruder to form hollow
cylindrical extrudates in the shape of so-called cartwheels
comprising five internal reinforcing spokes or stays leading from
the inner wall of the hollow cylinder to the center of the molding,
and was cut to the desired length of 3.5 to 5.5 mm by a cutting
device. The moldings were dried on a belt drier at 90.degree. C.
and were subsequently calcined for 6 hours at 900.degree. C.
The moldings obtained had the following physical and chemical
properties:
Molding dimensions: outside diameter (mm) 7.5 .+-. 0.5 height(mm)
4.5 .+-. 1 Wall thickness: 1.3 .+-. 0.05 Stay width: 1.3 .+-. 0.05
BET specific surface (m.sup.2 /g) 79 Pore volume (ml/g) 0.69 Bulk
density (g/l) 398 SiO.sub.2 content (% by weight) 99.9
Height/diameter ratio 0.6
EXAMPLE 2
85% by weight AEROSIL.RTM. 200
5% by weight methyl hydroxyethyl cellulose
5% by weight wax
5% by weight polyethylene glycol
were compacted in a kneader, with the addition of water which had
been made slightly alkaline with an aqueous alkaline ammonia
solution (15 ml of a 32% solution for a 2 kg batch). The kneaded
material was shaped in a single-screw extruder to form hollow
cylindrical extrudates in the shape of so-called cartwheels
comprising five internal reinforcing spokes or stays leading from
the inner wall of the hollow cylinder to the center of the molding,
and was cut to the desired length of 5.5 to 6.5 mm by a cutting
device. The moldings were dried on a belt drier at 90.degree. C.
and were subsequently calcined for 6 hours at 850.degree. C.
The moldings obtained had the following physical and chemical
properties:
Molding dimensions: outside diameter (mm) 6.0 .+-. 0.2 height (mm)
6.0 .+-. 0.5 Wall thickness: 0.95 .+-. 0.05 Stay width: 0.95 .+-.
0.05 BET specific surface (m.sup.2 /g) 148 Pore volume (ml/g) 0.75
Bulk density (g/l) 390 SiO.sub.2 content (% by weight) 99.9
Height/diameter ratio 1.0
EXAMPLE 3
85% by weight AEROSIL.RTM. 200
5% by weight methyl hydroxyethyl cellulose
5% by weight wax
5% by weight polyethylene glycol
were compacted in a kneader, with the addition of water which had
been made slightly alkaline with an aqueous alkaline ammonia
solution (15 ml of a 32% solution for a 2 kg batch). The kneaded
material was shaped in a single-screw extruder to form hollow
cylindrical extrudates in the shape of so-called cartwheels
comprising five internal reinforcing spokes or stays leading from
the inner wall of the hollow cylinder to the center of the molding,
and was cut to the desired length of 3.5 to 5.5 mm by a cutting
device. The moldings were dried on a belt drier at 90.degree. C.
and were subsequently calcined for 6 hours at 800.degree. C.
The moldings obtained had the following physical and chemical
properties:
Molding dimensions: outside diameter (mm) 7.5 .+-. 0.5 height(mm)
4.5 .+-. 1 Wall thickness: 1.3 .+-. 0.05 Stay width: 1.3 .+-. 0.05
BFT specific surface (m.sup.2 /g) 170 Pore volume (ml/g) 0.9 Bulk
density (g/l) 360 SiO.sub.2 content (% by weight) 99.9
Height/diameter ratio 0.6
EXAMPLE 4
85% by weight AEROSIL.RTM. 200
5% by weight methyl hydroxyethyl cellulose
5% by weight wax
5% by weight polyethylene glycol
were compacted in a kneader, with the addition of water which had
been made slightly alkaline with an aqueous alkaline ammonia
solution (15 ml of a 32% solution for a 2 kg batch). The kneaded
material was shaped in a single-screw extruder to form hollow
cylindrical extrudates in the shape of so-called cartwheels
comprising five internal reinforcing spokes or stays leading from
the inner wall of the hollow cylinder to the center of the molding,
and was cut to the desired length of 5.5 to 6.5 mm by a cutting
device. The moldings were dried on a belt drier at 90.degree. C.
and were subsequently calcined for 6 hours at 800.degree. C.
The moldings obtained had the following physical and chemical
properties:
Molding dimensions: outside diameter (mm) 6.0 .+-. 0.2 height(mm)
6.0 .+-. 0.5 Wall thickness: 0.95 .+-. 0.05 Stay widtb: 0.95 .+-.
0.05 BBT specific surface (m.sup.2 /g) 170 Pore volume (ml/g) 0.9
Bulk density (g/l) 350 SiO.sub.2 content (% by weight) 99.9
Height/diameter ratio 1.0
EXAMPLE 5
85% by weight AEROSIL.RTM. 200
5% by weight methyl hydroxyethyl cellulose
5% by weight wax
5% by weight polyethylene glycol
were compacted in a kneader, with the addition of water which had
been made slightly alkaline with an aqueous alkaline ammonia
solution (15 ml of a 32% solution for a 2 kg batch). The kneaded
material was shaped in a single-screw extruder to form hollow
cylindrical extrudates in the shape of what are termed miniliths as
shown in FIGS. 3 and 4, comprising nine passageway channels, and
was cut to the desired length of 4 to 5 mm by a cutting device. The
moldings were dried on a belt drier at 90.degree. C. and were
subsequently calcined at 800.degree. C.
The moldings obtained had the following physical and chemical
properties:
Molding dimensions: outside diameter (mm) 5.8 .+-. 0.2 height(mm)
4.5 .+-. 0.5 Wall thickness: 0.8 .+-. 0.05 Stay width: 0.8 .+-.
0.05 BET specific surface (m.sup.2 /g) 170 Pore volume (ml/g) 0.9
Bulk density (g/l) 350 SiO.sub.2 content (% by weight) 99.9
Height/diameter ratio 0.78
COMPARATIVE EXAMPLE 1
A palladium-gold-potassium acetate catalyst was prepared according
to Example 11 of EP 0 807 615 A1. The catalyst support which was
used was a molding according to Example 5 of EP 0 807 615 A1, but
which had the dimensions 8.times.5.times.3 mm (outside
diameter.times.height.times.inside diameter) and which had faceted
edges.
The concentrations of the impregnation solutions were selected so
that the finished catalyst contained a concentration of 0.55% by
weight palladium, 0.25% by weight gold and 5.0% by weight potassium
acetate.
In a first step, the support was first of all impregnated with a
basic solution of sodium hydroxide in water. The volume of the
aqueous NaOH solution corresponded to 50 percent of the water
absorption capacity of the dry support. After impregnation with
sodium hydroxide, the support was immediately impregnated, without
intermediate drying, with an aqueous solution of noble metals
comprising sodium palladium chloride and tetrachloroauric acid, the
volume of which likewise corresponded to 50 percent of the water
absorption capacity of the dry support.
After a holding time of 1.5 hours, during which the noble metal
compounds were hydrolyzed, the support particles were washed until
they were free from chloride. The support particles were dried and
were reduced at 450.degree. C. in the gas phase with forming gas
(95% by volume N.sub.2, 5% by volume H.sub.2). Thereafter, the
catalyst was impregnated with an aqueous solution of potassium
acetate and was dried again. Drying was effected in the gas phase
with nitrogen.
The sodium hydroxide concentration of the basic solution was
calculated so that a shell which contained noble metals and which
was formed on the support particles had a thickness <1.0 mm.
EXAMPLE 6
A palladium-gold-potassium acetate catalyst as described in
Comparative Example 1 was produced on the molding according to the
invention according to Example 1.
EXAMPLE 7
A palladium-gold-potassium acetate catalyst as described in
Comparative Example 1 was produced on the molding according to the
invention according to Example 2.
EXAMPLE OF USE 1
The activity and selectivity of the catalysts from Comparative
Example 1 and from Examples 6 and 7 were measured during a test
procedure which had a duration of up to 24 hours.
The catalysts were tested with the following gas composition: 75%
by volume ethylene, 16.6% by volume acetic acid, 8.3% by volume
oxygen, in an oil-heated tubular flow reactor (reactor length 710
mm, inside diameter 23.7 mm) at normal pressure and at a space
velocity (GHSV) of 400 h.sup.-1. The catalysts were investigated
over the temperature range from 120 to 165.degree. C., as measured
in the catalyst bed.
The reaction products were analyzed at the outlet of the reactor by
means of on-line gas chromatography. The space-time yield of the
catalyst in grams of vinyl acetate monomer per hour per kilogram of
catalyst (g VAM/(h.times.kg.sub.cat.) was determined as a measure
of the catalyst activity.
The carbon dioxide which was formed by the combustion of ethylene
was also determined and was employed for assessing the selectivity
of the catalyst.
The test results on catalysts from Comparative Example 1 and from
Examples 6 and 7 are presented in Table 1. The catalyst activity
and the catalyst selectivity of the catalyst according to
Comparative Example 1 were taken as 100 percent.
The results shown in Table 1 demonstrate that the catalysts
according to the invention, which are based on the moldings
according to the invention, exhibit a significantly higher activity
than that of the known comparative catalyst, whilst exhibiting a
comparable selectivity or even an improved selectivity.
TABLE 1 Activity Selectivity g VAM/(h .times. kg.sub.cat.) CO.sub.2
in the off-gas in % per unit area Catalyst temperature Catalyst in
% of Comp. Ex. 1 in % of Comp. Ex. 1 .degree. C. Comp. ex. 1 100
100(3.2) 146.4 B6 84.5 56.3 131.8 108.5 84.4 143.5 123.5 87.5 149.9
B7 104.1 65.6 132.2 135.3 75.0 144.7 153.7 103.1 152.0
* * * * *